My opinion...
Get an iron block engine. I don't believe that the displacement is very important since you are going to be using turbos, they will make up for a lot of displacement.
Remove the rotating assembly, put in main studs, rod bolts, and head studs. Have the rods honed as the new bolts will usually distort them. Same with the mains. Might as well balance it as well.
If you want to stick to your budget, go with a single turbo. Sucks, but twin turbos means 2 of everything. Even with ebay equipment you will blow that budget.
I don't know whether or not self-tuning efi will be able to adjust properly for turbos. I would use the original GM computer and haul it to the nearest tuner. It will be much cheaper and the original computer is more than capable.
Your money will be tied up in things like injectors to handle the power, ARP bolts/studs to help hold the engine together, and other little things. Use the turbo(s) to get your power. You can use Garrets turbo calculator to get an idea of what size of turbo(s) you will need, then purchase whichever brand suits your fancy. Quality and price tend to go hand in hand, however there is a large amount of people on the turbo forums who have $300 turbos pushing large amounts of power and, generally, reliable. Sure a couple have had things like oil seals and such go out, but for $300 you can get another.
I think your budget is do-able, but only if you do a single turbo with reliable components. Turbo is possible in the budget, but you'd have to deal with less reputable parts. Riskier, but not impossible.
Either way, that's my opinion. My build started the same, junkyard 6.0 with a single ebay turbo but snowballed. I've got a forged bottom end iron block 6.0 now, which will end up seeing a couple turbos down the road.
Good luck.
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